This invention relates to a Fexe2x80x94Ni alloy blank for use in making a shadow mask by fine etching, and more specifically to a Fexe2x80x94Ni alloy shadow mask blank which, when perforated by fine etching to form apertures through which electron beams pass, can improve the unevenness of aperture diameters due to the presence of irregular apertures and can provide electron beam apertures of uniform diameter and also relates to a shadow mask blank which has been formed with apertures for the passage of electron beams having improved unevenness of aperture diameters due to the presence of irregular apertures. The invention further relates to a method for manufacturing a Fexe2x80x94Ni alloy blank with such properties.
In the following description the concentrations of alloy components are given on the basis of mass proportions (%=mass percentage; ppm=mass proportion).
As material of shadow masks for color picture tubes, mild steel has been commonly used. The mild steel, however, presents a problem. Continuous use of a color picture tube increases the temperature of its shadow mask due to irradiation with electron beams. Consequent thermal expansion of the mask gradually brings the points of the screen that the electron beams strike through the mask out of register with the phosphor dots of the screen, causing color misregister or mismatching. The temperature rise of the shadow mask results from the fact that when a television is turned on, only less than one-third of the total amount of the electron beams passes the apertures of the shadow mask, the remainder of the electron beams striking the mask itself. More recently, therefore, a Fexe2x80x94Ni alloy of low thermal expansion coefficient known as xe2x80x9c36 (iron-36% nickel) alloyxe2x80x9d has come into use in the art of shadow masks for color picture tubes because of its merit in preventing color mismatching.
For the manufacture of a Fexe2x80x94Ni alloy blank for shadow mask, a Fexe2x80x94Ni alloy of a desired composition is melt-refined, for example, by vacuum melting in a vacuum induction melting (VIM) furnace or by secondary refining in a ladle furnace (LF). The molten metal is cast into an ingot, which in turn is forged or rolled by a blooming mill to a slab. The slab is hot rolled, descaled to remove oxide from the surface, repeatedly cold rolled and annealed for recrystallization, and, after the last recrystallization annealing, the rolled slab is finished by final cold rolling to a sheet of desired thickness in the range of 0.05 to 0.3 mm. The finally cold rolled sheet is slitted into blanks of desired width as shadow mask blanks. The blanks are degreased, coated with photoresist on both sides for patterning, exposed to light and developed to form a pattern, perforated by etching, and then cut to individual flat mask blanks. The flat mask blanks are annealed in a non-oxidizing atmosphere to impart press workability. (In the preannealing process this annealing is done on the finally cold rolled stock prior to etching.) The blanks are spherically pressed to the form of masks. Lastly, the spherically shaped masks are degreased, annealed in water vapor or combustion gas atmosphere to form a black oxide film on the mask surface. In this way shadow masks are manufactured.
For the purposes of this invention, the blanks to be etched for perforation after the final cold rolling for the passage of electron beams are collectively called shadow mask blanks. The term also encompasses the blanks, including flat masks, that have been perforated for the passage of electron beams and are yet to be press formed, as shadow mask blanks that have been formed with apertures for the passage of electron beams.
These shadow mask blanks are usually formed with apertures for the passage of electron beams by the well-known etching technique using aqueous ferric chloride. For the etching, photolithography is applied, and resist masks are formed on both sides of a blank, e.g., the mask on one side having a number of round openings 80 xcexcm in diameter and the corresponding points of the mask on the other side having round openings 180 xcexcm in diameter, and then aqueous solution of ferric chloride is sprayed over the both sides.
The etching provides the shadow mask blank a multiplicity of tiny apertures in a close arrangement. However, localized variation of etching conditions and other factors can result in unevenness of aperture diameters. If the unevenness is excessive, the shadow mask incorporated into a color picture tube can cause color mismatching and make the product defective. This unevenness of aperture diameters has hitherto been an important cost-raising factor as it decreases the yield in etch-perforation of shadow mask blanks for the passage of electron beams.
Various attempts have heretofore been made to control the unevenness of aperture diameters. From the material viewpoint, for example, Japanese Patent Application Kokai Nos. 5-86441 and 10-111614 propose precluding the unevenness through the control of the texture. They intend to secure the uniformity of etching by the texture control.
Our intensive research has, however, revealed that there is a phenomenon of unevenness of aperture diameter that cannot be coped with by the techniques of the prior art. FIG. 1 shows scanning electron micrographs (SEMs) of a xe2x80x9cnormal aperturexe2x80x9d formed by etching for the passage of electron beam and an xe2x80x9cabnormal aperturexe2x80x9d newly found to be a cause of unevenness of aperture diameters. (The shapes of the apertures formed upon etching of only one side were comparatively observed.) The abnormal aperture is characterized by rough wall surface compared with the normal aperture. The profile of the aperture is fringed and blurred with unusual etching, the diameter tending to be larger than the target value. The characteristic configuration of the abnormal aperture varies in degree with etching and other conditions; sometimes the surrounding wall is not roughened or the fringe or blur is not clearly observed. The unevenness of the aperture diameters with the formation of abnormal apertures has not been precluded by the prior art.
This invention is aimed at providing a shadow mask blank of Fexe2x80x94Ni alloy which, in perforation by etching to form apertures for the passage of electron beams, will not have unevenness in the diameters of the apertures due to the formation of abnormal apertures, even if the etching conditions are locally varied, and is also aimed at providing a method of manufacturing the blank.
We have made intensive study on the problems of the prior art from an entirely new, unique viewpoint and have found that, with a shadow mask blank of Fexe2x80x94Ni alloy which contains many minute inclusions, the perforation by etching scarcely causes the unevenness of aperture diameter due to the formation of abnormal apertures. Of the minute inclusions, particularly fine MnS has been found effective in controlling the unevenness of aperture diameter. In this case the MnS that proves effective in restricting the unevenness of the diameter of etched apertures for electron-beam passage is in the form of particles from 50 to 1,000 nm in diameter. The restricting effect was shown when the density (which means abundance, that is probability or frequency of existence) of MnS particles exceeded 1,500/mm2. For an elliptical, bar-like, or needle shape in the purposes of this invention, as shown in FIG. 2, the diameter of MnS particle is represented by the mean value of the shorter axis L1 and the longer axis L2.
Although the detailed mechanism by which MnS controls the unevenness of the diameter of etched apertures for the passage of electron beams is not yet clarified, it is presumed to be as follows:
A rolled blank of Fexe2x80x94Ni alloy according to this invention is usually etched to be a shadow mask, using an aqueous solution of ferric chloride. For that purpose a resist film is applied to the blank to cover the portions not to be perforated, so that only the portions to be perforated are exposed to the aqueous ferric chloride. If minute MnS particles are present in the portions to be perforated, they act as starting points of corrosion, accelerating the etching of the base metal. If no MnS is present in any of the portions to be perforated, all the portions are similarly etched, resulting in no unevenness of aperture diameter. In actual production on an industrial scale, however, difficulties are involved in reducing MnS and other inclusions to zero; in some portions to be perforated there are MnS particles that serve as corrosion-starting points with a certain probability. The portions to be perforated that have such corrosion-starting points initiate etching faster than the neighboring portions free from the corrosion-starting points, producing apertures with larger diameters. Since the portions to be perforated that have the starting points begin etching before the neighboring portions that do not have the starting points, the portions with the starting points electrochemically act as anodes, while the portions without the starting points act as cathodes. In this case the difference between the rates of corrosion becomes more pronounced and the difference between the diameters of etched apertures is greater too. If the blank contains minute MnS particles at a level beyond a certain density, the MnS particles are uniformly present in all the portions to be perforated, precluding any unevenness of aperture diameter.
With the blank which can form the xe2x80x9cabnormal aperturesxe2x80x9d as termed under this invention for the passage of electron beams, the uniformity of MnS throughout the material is lost because the MnS particles that serve as the starting points of corrosion are present at a level only below a certain density. With such a material, most of the portions to be perforated contain an average level of MnS, but there are (1) portions to be perforated that do not contain MnS; (2) portions that contain much MnS; and (3) portions in which the distribution of MnS is uneven. The portions to be perforated that contain MnS at levels different from the average differ in the etching rate, due to different degree of MnS contribution to etching, from the portions that contain MnS at the average level.
Consequently, abnormally corroded apertures characterized by their surrounding walls, aperture contours, aperture diameters, etc. are detected by observation under electron microscope. The abnormal apertures can be evaluated as a measure of unevenness of aperture diameters.
Thus, contrary to the established concept of the prior art, this invention intends to positively introduce minute MnS particles at the density greater than a certain level into a Fexe2x80x94Ni alloy base so as to eliminate or decrease the unevenness of diameters of etched apertures for the passage of electron beams. With this in view we have studied the means of introducing minute MnS into a Fexe2x80x94Ni alloy. As a result, it has now been found that mere adjustments of Mn and S concentrations are not satisfactory; rather, in a process for hot rolling a Fexe2x80x94Ni alloy slab, repeating cold rolling and recrystallization annealing, and finally cold rolling the resulting sheet to a desired thickness, it is necessary to optimize the thermal hysteresis of the material in the hot rolling and recrystallization annealing. This is because the solubility product ([%Mn]xc3x97[%S] where [Mn]: solid soluted Mn and [S]: solid soluted S) sharply decreases as the temperature drops in the temperature range from 600 to 1,200xc2x0 C. over which the Fexe2x80x94Ni alloy is heat treated. On the higher temperature side MnS dissolves in the Fexe2x80x94Ni alloy (hereinafter called xe2x80x9csolid solution or dissolutionxe2x80x9d) and on the lower temperature side MnS forms (hereinafter called xe2x80x9cprecipitationxe2x80x9d). We have accumulated fundamental data on the solid solution/precipitation behavior of MnS in Fexe2x80x94Ni alloys and have made extensive considerations. As a result, it has now been found that in the case of a Fexe2x80x94Ni alloy with a composition in conformity with this invention it is possible to set a temperature around 900xc2x0 C. as a boundary and deem the range of temperatures above the boundary as the MnS solid solution temperature region and the range of temperatures below the boundary as the MnS precipitation temperature range.
For commercial production of a Fexe2x80x94Ni alloy containing a desired proportion of minute MnS, it is necessary to inspect the MnS contained in the product at the site of manufacture for the purpose of the quality control of the product. The inspection of MnS particles ranging in diameter from 50 to 1,000 nm can be done using a transmission electron microscope. The method is cumbersome and not appropriate as an on-site inspection method, however. We thus have studied on the way of simply and conveniently determining the density of minute MnS particles. As a consequence, it has now come clear that when the surface of a Fexe2x80x94Ni alloy specimen is mirror polished and then immersed in a 3% nitric acid-ethyl alcohol solution at 20xc2x0 C. for 30 seconds to produce etched holes, a good correlationship is obtained between the density of MnS determined under a transmission electron microscope and the density of the etched holes from 0.5 to 10 xcexcm in diameter among the etched holes produced. The 3% nitric acid-ethyl alcohol solution is herein a mixture of 100 ml of ethanol having a purity of 99.5 vol % (JIS K8101 Special Grade) and 3 ml of nitric acid with a concentration of 60% (JIS K8541). FIG. 3 shows the results.
Observation of MnS under a transmission electron microscope is performed, over an area of 0.01 mm2, as follows:
(1) The surface of a specimen is electropolished at a constant potential. The electropolishing consists in polishing the specimen at the thickness corresponding to 5 coulomb/cm2 in a 10% acetylacetonexe2x88x921% tetramethylammonium chloride-methyl alcohol at a potential of +100 mV vs SCE. This electropolishing dissolves only the Fexe2x80x94Ni base surface, leaving undissolved inclusions protruding from the polished surface.
(2) When acetyl cellulose is applied to the electropolished surface and the resulting film is peeled off, the inclusions protruded from the polished surface now stick to the back side of the film.
(3) Carbon is evaporation-deposited onto the inclusions-sticking side of the acetyl cellulose film, and then the film is immersed in methyl acetate to dissolve the acetyl cellulose.
(4) The carbon film holding the inclusions is observed under a transmission electron microscope to inspect the states of the inclusions. At the same time, the compositions of the inclusions are identified by EDS and electron beam diffraction.
On the other hand, for the observation of the etched holes after the immersion in a 3% nitric acid-ethyl alcohol solution, an optical microscope was used and a dark field image of the corroded surface was photographed at 400 magnifications. From this photograph the number of etched holes with diameters between 0.5 and 10 xcexcm was counted. For the measurements of the etched holes an image analyzer was used to measure each surface area of 0.2 mm2. The etched holes were substantially spherically shaped, and their diameters were measured in the direction parallel to the rolling direction.
From FIG. 3 it is obvious that the number of MnS particles counted under a transmission electron microscope as the density of 1,500/mm2 corresponds to 2,000/mm2 in terms of the etched holes formed by the immersion in a 3% nitric acid-ethyl alcohol solution.
In view of the foregoing findings and considerations, this invention provides a shadow mask blank of Fexe2x80x94Ni alloy which exhibits excellent uniformity of diameter of apertures for the passage of electron beams when the apertures are formed by perforation with etching, consisting of, on the basis of mass percentage (%), from 34 to 38% Ni, from 0.05 to 0.5% Mn, from 4 to 20 ppm (mass proportion) S, and the balance Fe and unavoidable impurities or accompanying elements, provided that C is no more than 0.10%, Si is no more than 0.30%, Al is no more than 0.30%, and P is no more than 0.005%, wherein MnS inclusions from 50 to 1,000 nm in diameter are dispersed at the density of at least 1,500/mm2. Alternatively, it may conveniently be defined as a shadow mask blank of Fexe2x80x94Ni alloy which exhibits excellent uniformity of diameter of apertures for the passage of electron beams when the apertures are formed by perforation with etching, consisting of, on the basis of mass percentage (%), from 34 to 38% Ni, from 0.05 to 0.5% Mn, from 4 to 20 ppm (mass proportion) S, and the balance Fe and unavoidable impurities or accompanying elements, provided that C is no more than 0.10%, Si is no more than 0.30%, Al is no more than 0.3%, and P is no more than 0.005%, wherein etched holes from 0.5 to 10 xcexcm in diameter appear at the density of at least 2,000/mm2 when the blank surface is mirror polished and immersed in a 3% nitric acid-ethyl alcohol solution at 20xc2x0 C. for 30 seconds.
This invention also provides a method of manufacturing a Fexe2x80x94Ni alloy blank which comprises hot rolling a slab of Fexe2x80x94Ni alloy consisting of, on the basis of mass percentage (%), from 34 to 38% Ni, from 0.05 to 0.5% Mn, from 4 to 20 ppm (mass proportion) S, and the balance Fe and unavoidable impurities or accompanying elements, provided that C is no more than 0.10%, Si is no more than 0.30%, Al is no more than 0.3%, and P is no more than 0.005%; repeating cold rolling and recrystallization annealing, and, after final recrystallization annealing, finally cold rolling the rolled slab to a blank from 0.05 to 0.3 mm thick, through any of the process steps A to D mentioned below, wherein the blank either contains MnS inclusions from 50 to 1,000 nm in diameter dispersed at the density of at least 1,500/mm2 or has etched holes from 0.5 to 10 xcexcm in diameter appearing at the density of at least 2,000/mm2 when the blank surface is mirror polished and immersed in a 3% nitric acid-ethyl alcohol solution at 20xc2x0 C. for 30 seconds.
(Process Step A)
(1) In the course of hot rolling, working the slab in the temperature range of 950 to 1,250xc2x0 C. until the thickness is between 2 and 6 mm and, after the hot rolling, cooling the resulting rolled slab from 900xc2x0 C. down to 700xc2x0 C. at an average cooling rate set to 0.5xc2x0 C./second or below;
(2) In all of the recrystallization annealing runs, adjusting the temperature to 850 to 1,100xc2x0 C. and continuously passing the rolled material through a heating furnace filled with hydrogen or a hydrogen-containing inert gas, thereby adjusting the mean diameter of the recrystallized grains to 5 to 30 xcexcm; and
(3) Setting the reduction ratio of the cold rolling before the final recrystallization annealing to 50 to 85%, and setting the reduction ratio of the final cold rolling to 10 to 40%.
(Process Step B)
(1) In the hot rolling, working the slab in the temperature range of 950 to 1,250xc2x0 C. to a thickness of 2 to 6 mm;
(2) In the intermediate recrystallization annealing before the final recrystallization annealing, annealing the rolled material in a heating furnace filled with hydrogen or a hydrogen-containing inert gas to obtain recrystallized grains having a mean diameter of 5 to 30 xcexcm;
(3) In the final recrystallization annealing, holding the rolled slab in a heating furnace filled with hydrogen or a hydrogen-containing inert gas at an internal temperature of 650 to 850xc2x0 C. for 3 to 20 hours, thereby adjusting the mean diameter of the recrystallized grains to 5 to 30 xcexcm; and
(4) Setting the reduction ratio of the cold rolling before the final recrystallization annealing to 50 to 85% and setting the reduction ratio of the final cold rolling to 10 to 40%.
(Process Step C)
(1) In the course of hot rolling, working the slab in the temperature range of 950 to 1,250xc2x0 C. until the thickness is between 2 and 6 mm;
(2) In the intermediate recrystallization annealing before the final recrystallization annealing, holding the rolled material in a heating furnace filled with hydrogen or a hydrogen-containing inert gas at an internal temperature of 650 to 850xc2x0 C. for 3 to 20 hours to obtain recrystallized grains having a mean diameter of 5 to 30 xcexcm;
(3) In all the recrystallization annealing runs after the intermediate recrystallization annealing (2) above, passing the rolled material continuously through a heating furnace filled with hydrogen or a hydrogen-containing inert gas at an internal temperature of 850 to 1,100xc2x0 C., thereby adjusting the mean diameter of the recrystallized grains to 5 to 30 xcexcm; and
(4) Setting the reduction ratio of the cold rolling before the final recrystallization annealing to 50 to 85% and setting the reduction ratio of the final cold rolling to 10 to 40%.
(Process Step D)
(1) In the course of hot rolling, working the slab in the temperature range of 950 to 1,250xc2x0 C. until the thickness is between 2 and 6 mm;
(2) In all of the recrystallization annealing runs, annealing the rolled material in a heating furnace filled with hydrogen or a hydrogen-containing inert gas, thereby obtaining recrystallized grains from 5 to 30 xcexcm in mean diameter;
(3) Setting the reduction ratio of the cold rolling before the final recrystallization annealing to 50 to 85%, and setting the reduction ratio of the final cold rolling to 10 to 40%; and
(4) Performing, after the final cold rolling, annealing not involving recrystallization in a temperature range of 500 to 800xc2x0 C.
This invention further provides a shadow mask blank the above-defined Fexe2x80x94Ni alloy having apertures for the passage of electron beams formed by etching with reduced unevenness of aperture diameter due to the presence of abnormal apertures, wherein MnS inclusions from 50 to 1,000 nm in diameter are dispersed at the density of at least 1,500/mm2.